Modern semiconductor integrated circuits (ICs, commonly referred to as chips) are manufactured by means of a very complex process that may involve several hundred separate processing steps. Use of stringent quality control procedures and advanced manufacturing equipment is routine. Nevertheless, it is inevitable that the electrical characteristics of individual chips vary significantly from each other due to process variations. Further, the electrical characteristics of any particular integrated circuit vary as its operating conditions vary, for example, its temperature and power supply.
For some ICs, such variations can be plus or minus 15% for the resistance of a resistor and up to 20% for the capacitance of a metal oxide metal (MoM) capacitor. These variations directly affect the timing of the signals of the IC. Signal timing in digital ICs is generally synchronized by clock signals, which allows correct operation despite a certain amount of variation. In contrast, analog ICs are generally very sensitive to timing variations.
For example, in the case of an IC that implements a resistance capacitance (RC) low pass filter, the process variations of resistor and capacitor directly affect the cut off frequency. The cut off frequency is inversely proportional to the resistance capacitance product (called time constant) which can vary ±35%. A low pass filter allows the signal components with frequencies below cutoff frequency to pass and the filter stops or at least attenuates signal components higher than the cut off frequency. Many electronic devices that use low pass filters cannot tolerate a substantial variation in cut off frequency. Some electronic devices will not operate properly if a low pass filter does not meet stringent frequency requirements. Bandwidths of other types of resistor-capacitor based filters, such as high pass, band pass and band reject are associated with time constants and in certain applications must meet stringent frequency requirements.
Typically, an RC time-constant calibration circuit is single-ended while RC filters are fully-differential. As a result, the effects of parasitics in the filter are different from those in the calibration circuit. This causes a significant degradation in calibration accuracy. Thus, an accurate RC time-constant calibration circuit is needed.
Thus, there is a need for a system and method to accurately calibrate RC time-constants of integrated filters.